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'&" -n THE TIMES, WASHINGTON, SUNDAY, DECEMBER 30, 1900. -i si T The History CHEMISTRY. By PROF WILLIAM .RAMSAY, LL. D., D. Sc, Ph. D., F. R. S. (Gomrfufcf, 1HM, liy Tlie Sun Printiug and ruMtsliing issoctatton.) THE progress of tho science o chemistry forms one phase of the progress of human thought. While at first mankind was con tented to observe certain phenom ena, and to utilize them for industrial pur poses, if they were found suitable, "'philos ophers," as the thinking portion of our race loved to call themselves, have always attempted to assign some 'explanation for observed facts, and to group them Into slm. liars and dlsslmllars. It was for long Imag ined, following tho doctrines of the Greeks and of their predecessors, that all matter consisted of four elements or .principles, names which survive to this day In pop ular language. These were "fire," "air," "water" and "earth." It was not until the seventeenth century that Boyle In his "Sceptical Chymist" (1G61) laid the founda tions of the modern science by pointing out that It was Impossible to explain the exis tence of the fairly numerous chemical sub. stances known In his day, or the changes -which tbey can be made to undergo, by .means of the ancient Greek hypotheses re garding the constitution of matter. He laid down the definition of the modern meaning of the word "element;" he de clined to accept the current view that the properties of matter could be modified by Its assimilating the qualities of fire, air, earth, or water; and he defined an element as the constituent of a compound body. The first problem, then, to be solved, was to determine which of the numerous- forms of matter were to be regarded as elemen tary, and which tre compound, or com posed of two -or more elements in a state of combination; i-nd to produce such com pounds by causing tho appropriate ele ments to unite with each others One of the first objects to excite curiosi ty and interest was the air which sur rounds us, and In which we live and move and have our tlng. It was, however, en dowed with a semi-spiritual and scarcely corporeal nature. In the Ideas of our ances tors, for It does not affect the senses of Jght, smell or taste, and though It can be Salt, yet It eludes our grasp. The word "gas," moreover, was not invented until Van Helraont devised It to designate va rious kinds of "airs" which he had ob served. The important part which gases play in the constitution of many chemical compounds was accordingly overlooked; and. Indeed, It appeared to be almost as striking a feat of necromancy to produce a quantity of a gas of great volume from a small pinch Df solid powder as for a "Jinn" of enormous stature but of delicate texture to Issue from a brass pot, as re lated In the "Arabian Nights Entertain ments." Gradually, however, it came to bo recognized, not merely that gases have corporeal existence, but that they even -possess -weight. This, though foreshad owed by TonlcIUI, Jean Key and others, was first clearly proved by Black, profes sor of chemistry in Edinburgh, in 1732, through his masterly researches, as car bonic acid. The Ignorance of the material nature of gases and of their weight lies at the bot tom of the phlogistic theory, a theory de Vised by Stahl about the year 1CD0, to ac count for the phenomena of combustion and respiration and the recovery or "re fiuctlon" Df metals from their "earths" by heating with charcoal or nllled bodies. According to this inverted theory, a sub stance capable of burning was Imagined to contain more or less phlogiston, a princi ple -which it parted with on burning, leav ing an earth deprived of phljgiston, or "dephlosiacUed," behind if a metal. , -52ils eartn when heated with substances rich In phlogiston, such as ooal, wood, flour and similar bodies, recovered the phlogiston, which it had lost on burning, and with the added phlogiston Its metallic character. Other substances, such as phosphorous and sulphur, gave solids or acid liquids, to which phlogiston was not so easy to add; but even they could be re pblogistlcated. On this hypothesis, it -was the earths, and such acid liquids as sul phuric or phosphoric acids, which were the elements; the metals and sulphur and phosphorus were their compounds with phlogiston. The discovery of oxygen by Trlestley and by Schecle In 1771, and the explanation of Its functions by Lavoisier during the following ten years, gave their true mean ing to these phenomena. It was then rec ognized that combustion was union with oxygen; that an "earth" or "calx" was to be regarded as the compound of a metal with oxygen; that when a metal becomes tarnished, and converted Into such an earthy powder, it Is being oxidized; that this oxide, on Ignition with charcoal or carbon, or with compounds such as coal, flour, or wood, of which carbon Is a con. etituent, gives up Its oxygen to the car bon, forming an oxide of carbon, carbonic oxide on the one hand, or carbonic "acid" on the other, while the metal Is repro duced in its J'regullne" or metallic condi tion, and that tlw true elements are metals, carbon, sulphur, phosphorus, and similar bodies, and not the products of their oxidation. The discovery that air Is In the main a mixture of nitrogen, an inert gas, and oxygen, an active one, together with a small proportion of carbonic "acid" (or, as It is now termed, anhydride) a discov ery perfected by Itutherford, Black, and Cavendish and that water Is a compound with oxygen of hydrogen, previously known as Inflammable air, by Cavendish and by Watt, finally overthrew the theory of phlogiston; but at the beginning of this century It still lingered on, and was de fended by Priestley until his death in ISOi. Such, In brief, was the condition of chemical thought In the yer 18W. Schcele had died in ITiG, at the early age of forty four; Lavoisier was one of the victims of tho French Revolution, having been guil lotined in 1791; Cavendish had ceased to work at chemical problems, and was de voting his extraordinary- abilities to physi cal problems of the highest Importance, while living the life Ct an eccentric re cluse, and Priestley, driven by religious persecution from England to the more tol erant shores of America, was enjoying a peaceful old age, enlivened by occasional incursions Into the region of sectarian con troversy. The first striking discovery of our cen tury -was that of the comround nature of the alkalies and of -the alkaline earths. Tills discovery was made by Humphry Davy. Bom In Cornwall In 177S. he began the study of chemistry self taught, in X7S6; and 'in 17S9 he became director, of the 'Tncumatlc Institution." an undertaking founded by Dr. Eeddocs at Bristol for tho of the purpose of experiments on the curative effects of gases In generaL Here he nt once made his mark by tho discovery of the remarkable properties of "laughing gas," or nitrous oxide. At the same time he constructed a galvanic battery, and be gan to perform experiments with It In at tempting to decompose chemical com pounds by its means. In 1501 Davy was appointed professor of chemistry at the I Royal Institution, a society or club which had been founded a few years previously by Benjamin Thomson, Count Itumford, for the purpose of Instructing nnd amusing Its members with recent dis coveries in chemistry and natural philosophy. In 1S07 Davy applied his galvanic batter- to the decomposi tion of damp caustic potash and soda, using platinum poles. He was awarded by seeing globules of metal resembling mercury in appearance, at tho negative pole; and ho subsequently proved that these globules, when burned, reproduced the alkali from which they had been de rived. They also combined with "oxy murlatic acid," as chlorine (discovered by Scheele) was then termed, forming ordi nary salt, if sodium be employed, and the analogous salt, "muriate of potash," if f the allied methal potassium were subject ed to combustion. By using mercury as the negative pole, and passing a current through a strong solution of the chloride of calcium, strontium or barium, Davy suc ceeded In procuring mixtures with mer cury or "amalgams" of their metals, to which he gave the names calcium, stron tium, nnd barium. Distillation removed most of the mercury, and the metal was left behind In a state of comparative puri ty. The alkali metals, potassium and so dium, were"" found to attack glass, liberat ing "the Tjasls of the sllex," to which the name silicon has since been given. Thus nearly the last of the "earths" had been decomposed. It was proved that not merely were the "calces" of Iron, copper, lead and other well-known metals com pounds of the respective metals with oxy. gen, but Davy showed that lime, and Its allies, strontia and baryta, and even silica or flint, were to be regarded as oxides of elements of metallic appearance. To com plete our review of this part of the sub ject, suffice It to say that aluminum, a metal now produced on an industrial scale, was prepared for the first time in 1627 by Wohler, professor of chemistry at Gottln- gen, by the action of potassium on Its chlo ride, and alumina, the earthy basis of I clay, was shown to be the oxide of the metal aluminum. Indeed, the preparation of this metal In quantity Is now carried out at Schoffhausen-on-the-Ithlne, and at the Falls of Foyers, In Scotland, by elec trolysis of the oxide dissolved In melted cryolite, a mineral consisting of the fluo rides of sodium and aluminum by a method differing only in scale from that by means of which Davy Isolated sodium and potassium in 1S06. To Davy, too, belongs the merit of lav ing dethroned oxygen iiom its central po sition among the elements. Lavoisier gave to this Important gas the name "oxy gen," because he Imagined it to be the constituent of all acld3. He renamed the common compounds of oxygen In such a manner that the term oxygen was not even represented In the name only In ferred. Thus a "nitrate" Is &. compound of an oxide of nitrogen and an oxide of a metal; a "sulphate," of the oxide of a metal with one of the oxides of sulphur, and so on. Davy, by discovering the ele mentary nature of chlorine, showed first that it Is not an oxide of hydrochloric acid (or muriatic acid, as it was then called); tnd second, that the latter acid Is the com pound of the element chlorine with hydro gen. This he did by passing chlorine over white-hot carbon a substance eminently suited to deprive oxy-compounds of their oxygen and proving that no oxide of car bon is thereby produced; by acting on cer tain chlorides, such as these of tin or phosphorus with ammonia, and showing that no oxide of tin or phosphorus Js formed; and, lastly, by decomposing "mu riatic acid gas" (gaseous hydrogen chlo ride) with sodium, and showing that the only product besides common salt Is hydro gen. Instead, therefore, of the former theory that a chloride was a compound of the unknown basl3 of oxy-murlatlc acid with oxygen and the oxide of a metal, he Introduced the simpler and correct view that a chloride Is merely a compound of the element chlorine with a metal. In 1S13 ho established the similar nature of fluorine, pointing out that on the analogy of the chlorides It was a fair deduction that the fluorides are compounds of an undiscovered element, fluorine, with metals; and that hydrofluoric acid Is the true analogue of hydrochloric acid. The truth of this forecast has been established of recent years by Henri Molssan, who iso. lated gaseous fluorine by subjecting a mixture of hydrofluoric acid and hydrogen potassium fluoride contained In a platinum U tubo to the action of a powerful electric current. He has recently found that tho lube may be equally well constructed of copper; and this may soon lead to the In dustrial application of the process. The difficulty of Isolating fluorine Is due to Its extraordinary chemical energy; for there are few substances, elementary or com. pound, which resist the action of this pale yellow, suffocating gas. In 1S11 iodine, separated by Courtols from the ashes of sea plants, was shown by Davy to be an element analogous to chlorine. Gay-Lus-sac subsequently Investigated It and pre. pared many of its compounds; and In 1826 the last of these elements, bromine, was discovered In the mother liquor of sea salt by Balard. The elements of this group have been termed "halogens," or "salt producers." "While Davy was pouring his researches Into the astonished ears of the scientific and dilettante world, John Dalton, a Man chester schoolmaster, conceived a theory which has proved of tho utmost servlco to the science of chemistry and which bids fair to outlast our day. It had been no ticed by "Wenzel, by IUchter, by Wollas ton, and by Cavendish, toward the end of the last century, that the same compounds contain the same constituents In the same proportions, or, as the phrase runs, "pos sess constant composition." Wollaston, Indeed, had gone one step further, and had shown that when the vegetable acid, ox alic acid. Is combined with potash, it forms two compounds, in one of which the acid is contnlned in twice as great nn amount relatively to the potash as In the other. The names monoxalate and blnox- alate of potash were applied to these com- I pounds, to Indicate the respective propor tions of the Ingredients. Dalton conceived the happy Idea that by applying the an cient Greek conception of atoms to such facts the relative, weights of the atoms could be determined. Illustrating his views with the two compour-ls of carbon with hydrogen, marsh gas and defiant gas, and with the two acids of carbon, carbonic oxide, carbonic "acid,"" he re garded the former as a compound of one atom of carbon and one of hydrogen, and the second as a compound of one atom of carbon and two of hydrogen, and. similarly for the two oxides of carbon. Knowing the relative weights In which these ele ments enter Into combination, we can de duce the relative welghts'i'of tho atoms. -Placing the relative weight 'of an atom of hjdrogen equal to unity, we have: OIc- ' Car- ilarsh fiant borne Cos. Gas. Oilde. Carbon 6 6 Carbon S Hydrogen ..1 2 Oxygen .... 8 Thus tho first compound, marsh Car- !onIc Acid. 6 16 gas, was regarded by Dalton as composed of an atom of carbon In union with an atom of hydrogen; or to reproduce his symbols, as SO; while tho Mcond, okli.uit gas on this hypothesis, was a compound of two atoms of hydrogen with one of carbon, or GOO. Similarly tho symbols J O. and O 0 O were gl en to thu two couiKunds of carbon with oxygen. So water was as signed tho symbol 0 O. for Dalton Im agined It to be a compound of Dne atom of hydrogen with one of oxygen. Com pounds containing only two atoms were termed by him "binary;" those containing three, "ternary;" four, "quaternary," and so on. The weight of an atom of oxygen was eight times that of an atom of hydro gen; while that of an atom of carbon was six times as great as the unit. Uy assign ing symbols to the elements, consisting of the Initial letters of their names, or of the first two letters, formulas were developed. Indicating the composition of the com pound, the atomic weights of the elements being assured. Thus, Na O signified a compound of an atom of sodium (natrium) weighing 23 times as much as a similar atom of hydrogen, with an atom of oxy. gen, possessing eight times the weight of an atom of hydrogen. Therefore 21 pounds of soda should consist of 23 pounds of so dium In combination wllh eight pounds of oxygen, for, according to Dalton, each smallest particle of soda, contains an atom of each clement, and the proportion Is not changed, however many particles be con sidered. It has been pointed out by Judge Stallo, ! of Philadelphia, In his "Concepts of Phys ics," that such a hypothesis as that of Dalton is no explanation; that a fact of nature, as for example, the fact of simple and multiple proportions, is not explained by being minified. Allowing the general truth of this statement. It Is nevertheless undoubted that chemistry owes much to Dalton's hypothesis; a lucky guess at first, it represents one of the fundamental truths of nature, although Its form must be somewhat modified from that in which Dalton conceived it Dalton's work was first expounded by Thomas Thomson, pro fessor at Glasgow, In his "System of Chemistry," published In 1803, and subse quently In Dalton's own "New System of Chemical Philosophy," the three volumes of which were published In 1S0S, In 1S10, nnd In 1827. The determination of these "constants of nature" was at once followed out by many chemists; Thomson among the first But chief among tho chemlst3 who have pursued this branch of work was Jacob Bcrzellus, a Swede, who devoted 'his long life (1779-1518) to the manufacture of com pounds, and to the determination of their composition, Dr as it Is still termed, the determination of the "atomic weights" more correctly, "equivalents" of the ele ments of which they are composed. It Is to him that we owe most of our analytical methods, for, prior to Ills time, there -were few, if any, accurate analyses!. Although Lavoisier had devised a method for tho analysis of compounds of carbon, viz, by burning the organic compounds in an at mosphere of oxygen contained in a bell Jar over mercury, and measuring tho voir ume of carbon dioxide produced, as well as that of the residual oxygen, Berzclius achieved tho same results more accurate ly and more expeditiously by heating the substance, mixed with chlorate of po tassium and sodium chloride, and then es timating the hydrogen as well as the car bon; this process was afterward perfected by Llebig. Berzellus, however, was able to show that compounds of carbon, like those of other elements, were Instances of combination In constant and In multiple proportions. In 1815 two papers were published in the "Annals of Philosophy" by Dr. Prour, which have hajl much Influence on the progress of chemistry. They dealt with the figures which were being obtained by Thomson, Berzellus, and others, at that Ume supposed to represent the "atomic weights" of the elements. Prout's hypo thesis, based on only a few numbers, was that the atomic weights of all elements were multiples of that of hydrogen, taken as unity. There was much dispute regard ing this assertion at the -time, but as It was contradicted by Berzellus" numbers, the balance of opinion was against It. But about the year JSiO Dumas discovered an error In the number (12.12) given by Ber zellus as the atomic weight of car Ion; and with his collaborator, Stas, undertook the redetermination of the atomic weights of the commoner elements, for example, car bon, oxygen, chlorine, and calcium. This line of research was subsequently pursued alone by Stas, whose name will always be remembered for the precision and accu racy of his experiments. At first Dumas and Stas Inclined to the view that Prout's hypothesis was a just one, but it was com pletely disproved by Stas subsequent work, as well as by that of numerous other observers. It Is nevertheless cu rious that a much larger proportion of the atomic weights approximate to whole numbers than would bo foretold by the doctrine of chances, and perhaps the last has not been heard of Prout's hypothesis, although In Us original crude form It Is no longer worthy of credence. One of the most noteworthy of the dis coveries of tho century was made by Gay Lu3sac (1778-1830) In the jear 1808. In con junction with Alexander von Humboldt, Gay-Lussac had rediscovered about three years before what had previously been es. tabllshcd by Cavendish, namely, that as nearly as possible two volumes of hydro gen comolne wllh one volume of oxygen to form water, tho gases havlns been measured at the same temperature and pressure. Humboldt suggested to Gay Lussac that It would be well to Investigate whether similar simple relations exist be tween tho volumes of other gaseous sub stances when they combine with each other. This turned out to be the case; It appeared that almost exactly two volumes of, carbonic oxide unite with one volume of oxygen to form carbon dioxide; that equal volumes of chlorine and hydrogen unite to form hydrochloric acid gnu; that two volumes of ammonia gas consist of three volumes of hydrogen In union with one volume of nitrogen, and so on. From such facts, Gay-Lussac was ltd to make the statement that: Tho weights of equal volumes of both simple and compound gases, and thereforo their densities, are proportional to their empirically found combining weights, or to rational multl- pies of the latter. Gay-Lussac recognized thl3 discovery of his to be a support for the atomic theory; but It did not accord with many of the then received atomic weights. The assumption 'that equal vol umes of gases contain equal numbers of particles, or as they wero teimed by him", molecules lntegrantes, was made In 1S11 by Avogadro, professor of physics- at Turin (1776-1S36). This theory, which has proved of the utmost Importance to the sciences both of physics and of chemistry, had.no doubt occurred to Gay-Lussac, and had been rejected by him for the follow ing reasons: A certain volume of hydro gen, say one cubic Inch, may be supposed to contain an equal number of particles (atoms) as an equal volume of chlorine. Now, these two gases unite in equal vol umes. The deduction appears so far quite legitimate that one atom of hydrogen has combined with odc atom of chlorine. But the resulting gas occupies two cubic inches, and must therefore contain the same number of particles of hydrogen chloride, tho compound of the two ele ments, as one cubic inch originally con tained of hydrogen, or of chlorine. Thus we have two cubic inches containing, of uncomblned gases, twice as many par ticles as Is contained In that volume after combinations. Avogadro's hypothesis solved the difficulty. By premising two different orders of particles, now termed atoms and molecules, the solution was plain. According to him, each particle or molecule of hydrogen Is a complex, and contains two atoms; the same Is the case with chlorine. When these gases combine, or rather react, to form hydrogen chloride, the phenomenon is one of a change of partners; the molecule, the double atom, of hydrogen splits; the same is the case with the molecule of chlorine; and each liberated atom of hydrogen unites with a liberated atom of chlorine, forming a com pound, hydrogen chloride, which equally consists of a molecule, or double atom. Thus two cubic Inches of hydrogen chlo- -ride consists of a definite number of mole cules, equal In number to those contained In a ruble Inch of hydrogen plus those contained In a cubic Inch of chlorine- The case Is precisely similar. If other com pounds of gases be considered. Berzellus was at first Inclined to adopt this theory, and Indeed went so far as to change many of his atomic weights to make them fit It. But later he somewhat withdrew from his position, for It ap peared to Jilm that It wis hazardous to extend to liquids and solids a theory which could bo held only of gases. Avo gadro's suggestion, therefore, rested lif abeyance until the publication. In IS3S, by Cannlzzaro, now professor of chemistry In Home, of an essay In which all the argu ments' In favor of the hypothesis were col lected, and stated In a masterly manner. It will be advisable to revert to this hy pothcsls at a later point, and to consider other guides for the determination of atomic weights. In 1S13, Dulong (17S3-1S33), director of the Ecole Polv technique at Paris, and Petit (1791-1820) professor of physics there, made the discovery that equal amounts of heat are required to raise equally the tempera ture of solid and liquid elements, provided quantities are taken proportional to their atomic weights. Thus to raise the temper ature of 56 grammes of Iron through one degree requires approximately the same amount of heat as Is required to raise through .one degree 32 grammes of sul phur, C3.5 grammes of copper, and so on; these numbers representing the atomic weights of the elements named. In other words, equal numbers of atoms have equal capacity for heat. The number of heat units or calories (one calory Is the amount of heat required to raise the -temperature of 1 gramme of water through 1 degree centigrade) which is necessary to raise the .atomic weight expressed in grammes of any solid or .liquid element through 1 de gree centigrade la 'approximately fc.2; it varies between 5.7 and C.C In actual part. This affords a. means of" determining the true value of the atomic weight of an ele- jnent, as the following -example will show: The analysis of the only compound of zinc and chlorine chows that It contains 47.15 per cent of zinc and 52.IG per cent of chlo rine. Now, one grain of hydrogen com bines with 33.$ grains of chlorine to form 26.5 grains of hydrogen chloride; and as already remarked, one volume of hydro gen and ono volume of chlorine combine, forming two volumes of hydrogen chlo ride. Applying Avogadro's hypothesis, one molecule of hydrogen nnd one mole cule of chlorine react to yield two mole cules of hydrogen chloride; and as each molecule is supposed to 'consist In this case of two atoms, hydrogen-chloride con sists of one atom of each' of Its constitu ent elements. The amount of that ele ment, therefore, which combines with 33.5 grains of chlorine may give the numerical value of the atomic weight of the clement. If the compound contains one atom of each element; In that case the formula of tho above compound would be zinc, and the atomic weight of zinc, 32.7; but If the formula is ZuCi tho atomic wcight-of zinc would bo 32,7x2: If ZuCl;, 32.7x3, and so on. The spcclflc heat of metallic zinc enables this question to be solved. Tor It has been found experimentally to be about 0.093: and 6.20.08303.2, a close approximation to 32.7x263.4. The conclusion Is therefore drawn that zinc chloride Is composed of one atom of zinc in combination with two atoms of chlorine, that tho atomic weight of zinc Is 65.4, and that tho molecular weigh t of zinc chloride is 05.4(33.5x2-13IS.4. Inasmuch as the relu tlvo weight of a moki 'lo of hydrogen Is 2 (that of an atom being ;, zinc chloride In the gaseous suite should bo 1.10.4-5-2 k.2 times that of hydrogen, measured at tho same temperature and pressure. This has been found experimentally to be the case. The methods of determining the vapor densities, or relative weights of vapors, aio three in number; the first method, due to Dumas (1S27) consists In vaporizing the substaiico In question In a bulb of glas3 or of porcelain, at a known temperature, closing the bulb while still hot, and weigh ing It after it Is cold. Knowing the ca paclty of the bulb, the weight of hydro gen necessary to fill it at the desired tem perature can be calculated, and the densi ty of the vapor thus arrived at. A second method was devised by Gay-Lussac and perfected by A. W. Hofmann (15CS); and a third, prefcrablo for Its simplicity and case of execution. Is due to Victor Meyer (1S81). In 1S38, ns already remarkPd, Cannlzzaro showed the connection between thesa known facts, and for tho first time atten tion was called to the true atomic weights, which were, up to that time, confused with equlalents, or weights of elements required to replace one unit weight of hy drogen. These wero generally regarded as atomic weights by Dalton and his contem porarlcs. Some exceptions had been observed to the law of Dulons and Petit, viz: beryl Hum or glucinium, an element occurring in emeralds; boron, of which borax Is a compound; silicon, the component of quartz and flint, and carbon. It was found by Weber that at high tempera tures, the specific heats of these elements aro higher, and, the atomic heats approx imate to the number of C.2; but this be havior Is not peculiar to these elements, for It appears that the spcclflc heat of all elements Increases Trith rise of tempera ture. A certain number of exceptions have also been noticed to the law of Gay-Lus sac, which may be formulated: ihe molecu lar weight of a, compound In n gaseous state is twice its density referred to hy drogen. Thus, equal volumes of ammonia and hydrogen chloride unite to form am monlum chloride. It was to be expected that the density should be half tho mo lecular weight, thus: (N IIiII UNH,C1 ; ;.nd 53JS226.73-denslty 143X133 5) 513. But tho density nctually found Is only half that number, viz, 13.37; and for long this and similar cases were supposed to be exceptions to the law of Gay-Lussac, viz, that equal volumes of gases at tho same pressure expand equally for equal rise of temperature. In pthcr instances the grad ual decrease In density with rise of tem perature can be followed, as with chloral hydrate, the products of which are chloral and water. It was recognized by St. Claire Dcvllle (1837) that the decrease In density Df such mixtures of gases was due, not to their being exceptions to Avogadro's law, but to the gradual decomposition of the com pound body with rise of temperature. To this gradual decomposition he gave the name dissociation. This conception has proved of the utmost Importance to tho science, as will be seen In the sequel. To take the above lnstar.ee of ammonium chloride. Its abnormal density Is due to its dissociation into ammonia and hydro gen chloride; and the gas which is ob tained on raising Its temperature consists. not of gaseous ammonium chloride, but of a mixture of ammonia and hydrogen chlo ride, which, as Is easily seen, occupy, when separate, twice tho volume that would be occupied by the gaseous com pound. Of recent years It has been shown by Brereton Baker that It perfectly free from moisture, ammonium chloride gas ifies as -such, and that its density in tho state of vapor is In fact 26.73. The molecular complexity of gases has thus gradually become comprehended, and the truth of Avogadro's law has gained acceptance. And as a means of picturing the behavior of gaseous molecules the kl.ictlc theory of gases has been devised by Joule. Clauslus. Maxwell Thomson (Lord Kelvin), and others. On the as sumption that the pressure of a gas on the walls of the vessel which contains it ls due to the continued Impacts of its molecules, 'and that the temperatunTof a. gas Is represented by the product of tho mass of the molecules, or the square of their velocity, It has been possible to offer a mechanical explanation of Boyle's law, that at constant temperature the volume of a gas diminishes In proportion as the pressure increases; of Gay-Lussac's law, that all gases expand equally for equal rise of temperature, provided pressure Is kept constant; the condition being that equal volumes of gases contain equal numbers of molecules. A striking support Is lent to this chain of reasoning by the facts discovered by Thomas Graham (1S03-1SC9). professor at University College, London, and subsequently master of the Itoyal Mint. Graham discovered that the rates of diffusion of gases Into each other Is Inversely as the square roots of their densities. For instance, tho density of hy drogen being taken as unity, that of oxy gen is 16 times as great; If a vessel con- gen is id times u bici, 4. icaoci mu talning hydrogen bo made to communlcats with one containing oxygen, the hydrogen will pass into tho oxygen and mix with It; and conversely, the oxygen will pass Into the hydrogen vessel. This Is due to the Intrii.slc motion of the molecule of each gas. And Graham found experimentally that for each volume of oxygen which tn ters tho hydrogen vessel four volumes of hydrogen will enter the oxygen vesseL Xow, 43j'16; and as these masses are rela tively 1 and 16, and their temperatures are equal, the square of their velocities are respectlvelyl and 16. The question of the molecular complex ity of gases being thus disposed of, it re mains to be considered what are the rela tive complexity of liquid molecules. The 'answer Is Indicated by a study of the ca pillary phenomena of liquids, one method of measuring which Is the height of their ascent in narrow or capillary tubes. It Is Impossible In tho space at our disposal tc enter Into detail as to the method and arguments necessary; suffice It to say that the Hungarian physicist Eotvos was the first to Indicate tho direction of research, and that Bamsay and Shields suceeded In proving that the complexity of the mole cules of most liquids is not greater than that of the gases which they form on be ing vaporized; and also that certain liquids, e. e- water, the alcohols, and ether llaulds, are more or less "asso ciated;" 1. e., their molecules occur in coupllces of two, three, four, or more, and as the temperature Is raised the complex- lty of molecular structure diminishes. As regards the molecular complexity of solids, nothing definite Is known, and. moreover, there appears to be no method capable of revealing It. While the researches of which a short account has now been given have led to knowledge regarding the nature of mole cules, the structure of the molecule has excited Interest since the ear'y years of the century, and its Investigation has led to Important results. The fact of lhe de composition of acidified water by an elec tric current, discovered by Nicholson and Carlisle, and of salts Into "bases" and "acids" by Berzellus and Hisinger in 1853, led to the belief that a close connec tion exists between electric eneigy, or, as It was then termed, "electric force," and the affinity which holds the constituents of chemical compounds In combination. In 1S07 Davy propounded the theory that all compounds consist of two portions, one electro-positive and the other ilectro-ncg. atlve. Thi3 Idea was the result of experi ments on the behavior of substances such, for example, as copper and sulphur. If portions of these elements be Insulated and then brought Into contact they be come oppositely electrified. Tho degree of electrification Is Intensified by rise of tem perature until, when combination ensues, the electrification vanishes. Comblratlon, therefore, according to Davy, is concur rent with the equalization of potentials. In 1S12, Berzellus brought forward an electro-chemical theory which for the follow ing twenty years was generally accepted. His primary assumption was tl at the atoms of elements, or. In certain cases, groups of atom3, aro themselves electri fied; that each atom, or group of atoms, possesses two poles, one positive, the other negative; that the electrification of ono of these poles predominates over that of the other, so that the atom or group Is Itself, as a whole, electropositive, or electro-negative; that combination ensued be tween such oppositely electrified bodies by the neutralization, partial or complete, of their electric charges; and lastly, that the polarity of an element or group could bo determined by noting whether the element or group separated at the positive or at tho negative polo of tho galvanic battery, or electrolysis. For Berzellus, oxygen was the most electro-negatlvo and potassicm the most electro-positive of the elements, the bridge between the "non-metals" and the "metals" being hydrogen, which, with nitrogen, forms a basic, or electro-positive, group, whllo with chlorine, etc., It forms electro-negative groups. The fact that an electric current splits compounds in solution Into two portions, led Berzellus to devise his "duallstlc" system, which Involved the assumption that all com pounds consist of two portions, one electro-positive, the other electro-negative. Thus sulphate of magnesium and potas sium was to be regarded as composed of electro-positive potassium sulphate In combination with electro-negative magne sium sulphate; the former In its turn con sisted of electro-negative sulphur trloxlde (SOa) In combination with electrc-posltlvo otldo of potassium (Kip); whllo oach of these proximate constituents of potas sium sulphate were themselves composed of the electro-negative oxygen In combina tion with electro-posltlvo sulphur, or potassium. On contrasting sulphur with potassium, however, the -former was con sidered more electro negative than the latter? so that the group bOj.is a whole was electro-negative, while ICjO was electro-positive. The symbols given above, which aro still In universal use, wero also de vised by Bencllus for the purpose of Illus trating and emphasizing his vlew3. These views, howovcr, met with little acceptance at the time in England. Lavoisier's Idea, that oxygen was the necessary constituent of all acids, began about this time to lose ground. For Davy had proved the elementary nature of chlo rine; and hydrochloric r.cld, one of the strongest, was thus seen to contain no oxygen, and Davy expressed the view, founded on his observation, that iodic "acid," UOi was devoid of acid properties, until dissolved In w?ter, and that tho essential constituent of all acids was hy drogen, not oxygen. The bearing of this theory on the duallstlc theory Is, that while, e. P-, sulphuric acid was regarded by lierzellns as tOj, containing no hydro gen, and was supposed to be separated as such at the positive pole of a battery, Davy's suggestion led to the opposite con clusion that the formula of sulphuric acid Js HjSOj. and that, by tho current. It Is re solved Into Hs and SO. Faraday's electro lytic law, that when a current Is passed through electrolytes In solution the ele ments are liberated in quantities propor tional to their equivalents, led to the abandonment of the duallstlc theory. For when a current Is passed In succession through acidified water, fused lead chla- ; f lue anu a solution oi potassium suipnaie. ; lne Quantities oi hydrogen ana oxygen ! tTOm tn water, of lead and chlorin from , lhe leaa chloride, and the potassium of the sulphate are In accordance with Faraday's law. But In addition to the potassium. ther0 ,s "Crated at the same pole an equivalent of hydrogen. Now, if Berze llus theory be true, the products should be SO, and K.O, but If tho opposite view lie correct, then K- Is liberated first and Jiy its subsequent action on water It yields potash, and Its equivalent of hydrogen. This was pointed out first by Danlell, pro fessor at King's College, London, and It was regarded a3 a powerful argument against Berzellus' system. In 1833, too, Graham Investigated the phosphoric acids, and prepared the salts of three, to which he gave the names ortho-, pyro-, and mcta-phosphorlc acids. To understand the bearing" of this on the doctrine of dualism It must bo remembered that IOr, pentoxlde of phosphorus, was at that date named phosphoric acid. WTicn dissolved In water it reacts with bases, forming nlts the phosphates. But the quantity of water necessary was not then consid ered essential; Graham, however, showed that there exist three series of salts ono set derived from VjOj.31IJO; one from .... " ' IVWbsO, and a third f rom P-0-,II.O. Ills way of stating the fact was that water could play the parrof a bese; for example, the ordinary phosphate of commerce pos sessed, according to him, tho formula I'iO2Xa.O,II.O; two-thirds of Jho "water of consUtutlon" being replaced by oxide of ovum,,, jicuig, men uruivssur ui oiesaun (1S03-1S73), founded on these and on similar observations of his own the doctrine of polybaslc acids acids in which one, two, three, or more atoms of hydrogen wero replaceable by metals. Thus, Instead of writing-, as Graham did, P-Oi.2NajO.H-0. he wroto FOjXajH; and for orthophosphorlc acid PO,H Tho group of atoms (TO,), therefore existed throughout the whole se ries of orthophosphate3, and could exist In combination with hydrogen, with hy drogen and metals, or with metals alone. Similarly the group (P,Oj) was characteristic of pyrophosphates and (POJ of metaphos pbates,forIVOi2II,0(rsOj)l',;and PsOH,0 2(POJU, The first clear Ideas of the structure of the- molecule were, however, gained Crom the study of the compounds of carbon. It was difficult to apply the duallstlc theory to them, for few of them are- electrolytes and therefore their products of electro lysis, being non-existent, could not be classslfied. Nevertheless Gay-Lussac re garded alcohol, C.II4O, as a compound of CjII ethylene, and ILO, water, and oxalic acid (an hydrous), CjO-, as one of COj with CO. TI10 discovery of "isomeric compounds," i. e., of compounds which possess the same ul timate formula and yet differ entirely in their properties, forced upon chemists the necessity of attending to the structure of the molecule; for only by such a supposi tion could the difference between two Iso meric bodies be explained. In 1S23 Llebig discovered that silver fulminate and sil ver cyanate both possessed the empirical formula AgCNO; In 1825 this was followed by the discovery by Faraday that oil gas contains a hydrocarbon Identical In com position with ethylene, C-IL. yet differing from It In properties; and In 1S20 Wohler, professor In Gottlngen (1S09-1SS2), discov ered that urea, a constituent of urine. could be produced by heating ammonium cyanate. NII,CNO, a. substance of tho same formula. It therefore became clear that the Identity of a compound must depend on some other causo than Its ultimate composition. In 1S33 Llebig and Wohler took an Im portant rtep In elucidating this question by their Investigations on benzoic acid an acid obtainable by distilling a resin named gum benzoin. They showed that this acid. CjIIsOa coald bo conceived as con sisting of tho group C;ir;0, to which they gave tho name "benzoyl," In combination with OH; that benzoic aidchjdt, CjIUO. might bo regarded as Its compound with hydrogen; that It also formed compounds with chlor ine and bromine and sulphur, and re placed hydrogen Is ammonia (CjIUO.NH-). They termed this group, benzoyl, a "com pound element" or a "radical." This re search was followed hy ono by Robert Bunsen, prpfessor at Heidelberg, born In 1811, and recently (1SS3) dead, which bore reference to cacodyl, a compound of ar senic, carbon, and hydrogen. In which tho idea of a radical was confirmed and amplified. The Idea of a radical having thus be come established. Jean Baptiste Andree uumas, proiessor in i-ans iisuo-issu), pro pounded the thory of "substitution," 1, e., that an element such as chlorine or oxy gen (which, be it noticed. Is electro-negative on Berzellus' scale) could replace hydrogen In carbon compounds, atom for atom, tho resulting compound belonging to the same "typo" as the one from which It was derived. And Laurent, warden of the mint at Paris (1S37-1S33), and Gerhardt, professor at Montpelier and at Strassburg (1S16-18J6), emphasized tho fact that one element, bo It what It may, can replace another without fundamentally altering Its chemical character, and also that an atom of hydrogen can be replaced by a group of atoms or radical, behaving for the occasion like the atom of an clement. It is to Laurent and Gerhardt that we ovrt the definition of an atom "the smallest quantity of an clement which can be pres ent In a compound;" an equivalent "that weight of an element which combines with or replaces one part by weight of hydro gen;" and a molecule "the smallest quan tity which can exist In a free state, whether of an element or a compound." They recognized, too, that a molecule of hydrogen, chlorine, etc., consists of two atoms. In 1519, Wurtz,' professor In Paris (1S17- 1SS4), and Hofmann, then professor In th College of Chemistry In London, after ward at Berlin (1S1S-1S92), dlscoveted series of compounds ullleil to ammonia, KHJ In which one or more atoms of hydrogen, were replaced by a group or radical, suih. a? methyl (CIIJ. ethyl (CIIj). or ph-nyl (CIU WurU referred such compounds to th ammonia "type." They all resemble am monla in their physical properties smelL taste, etc as well as In their tor-f uniting with adi. to form salts resem. Wing ammoniam chloride MI, CI), and other ammonium compounds. Shortly after ward Williamson, professor at University College, London (1824), added the "water type," In consequence of his researches on "mixed ethers" bodies in which tha hydrogen of water might be regarded as replaced by organic radicals. Thus we have the scries: II. O.H.; CH 0. n.; CIL, O. CHi; and Nil,; SD n: NH(CH,),; and N(Cn,),; tho first representing . compounds following- tha water typo, the latter the am monla type. This suggestion had been previously made by Laurent In 1S16. But Williamson extended his views to inorganic compounds; thus, sulphuric acid was represented as constructed on thcdonblo water type HO. SO OH. being de rived from ir. O. 0?, ,H) 6. If, the two hydrogen atoms enclosed in parentheses being replaced by the radical FOj To tlicso types Gerhardt added the hydrogen and hydrogen chloride types, H.IL and JI.CI: and later Kekule, professor In Bonn (1829), nclchd the .Marsh gas type C(U). Tho next Important step was taken by Frankland. professor In the Royal School of Mines, London; his work, however, had been an ticipated -by Cunn Brown (professor at Edinburgh University) In a pamphlet even yet little known. It was to attribute- to elements one or more powers of combina tion. To these he gave the name "val ency," and the capacity of possessing valency was called "quantlvalence." Thus hydrogen was taken as a "monad," or monovalent. Chlorine, because it unites with hydrogen atom to atom, is also a monad. Oxygen, having the power to com bine with two atoms of hydrogen, was termed a dyad, or divalent; nitrogen a triad, or trlvalent; carbon a tetrad, or tetravalent, and so on- This Is evident from inspection of the formulae of their compounds with hydrogen, thus: H-fl , H- 0-H , H-NCJJ 5 IX Instances of penta, hexa, and even heptav valency are not wanting. This was the key to unlock the structur& of chemical compounds; and Frankland's views, just stated, are still held by chem ists. The determination of the constitu tion of compounds, chiefly thoseof car btn, occupied the attention of chemists almost exclusively until 1SS0. The plan of action Is much the same as that of a me chanician who wishes to Imitate a com plicated mechanism. He must first dissect it Into groups of mechanical contrivances: these are next constructed; and they are finally built together Into the complete machine. In certain cases the atoms of carbon are arranged In "chains," as, for example, in pentyl alcohol: HjC C C C C O-U n.n. nsHs each atom being tetrad, and Its "affini ties," or powers of combination, saturated jlther with hydrogen or with those of neighboring atoms of carbon; In othera they are in the form of a "ring," as In benzene, the formula of which was first suggested by Kkule: viz: H H H H or In both, as In ethyl benzene - L h Hf C c C-C-CH h a H H One or more atoms of nitrogen, or of oxygen, may form part of the circle, as In pyridine. H H r. cCH H. H H H ,C C I c c H H and furfurane. and so on. By means of conceptions such as these many interesting compounds have been built up out of the elements which they contain; e. g., urea and uric acid, constituents of urine; theo-bromlne and caffeine, the essential principles of cocoa and tea; alizarine and Indigo, val uable dye stuffs; and several of the alka loids, bitter principles contained In plants, of srsat medicinal value. They have led, too, to the discovery of many brlllant colors, now almost uni versally employed, to the exclusion of those less brilliant, because less pure, derived from plants, and In one or two cases from animals; the manufacture of gun cotton, dynamite, and similar high explosives; and to the development of the candle industry, the sugar manufacture. to Improvement in tanning. In brewing, and in the preparation of gas and oils for illuminating purposes. In short, It may be said that tha Industrial progress of the latter half of the century has been due to the theoretical views of which a short sketch has Just been given. Such formulae, however, can evidently not represent the true constitution of mat ter. Inasmuch as tho atoms are Imagined to lie on a plane, whereas It Is evident that they must occupy space of three di mensions and possess the attributes of solidity. The conception which led to tho formulation of such vle-vs was due first to Pasteur. In his later years director of tha Institute known by bis name at Paris, and more directly to LeBel, and van t' Hoft, now professor at Berlin, Independently of each other. In ISIS Pasteur discovered that it was possible to separate the twe varieties of tartaric acid from each other, and that that one which rotated the plana of polarized light to the right gave crys tals with an extra face, unsymmetrically disposed with regard to the other faces of tho crystal. The variety, tho solution of which In water was capable of producing left-handed rotation, also possessed a simi lar face, but so placed that its reflection In a mirror reproduced the right-handed 1 variety. Pasteur also showed that a mix ture of these acids gave crystals not char-, acterlzed by an Unsymmetrically placed face; and also that the solution was with out action on polarized light. These ob servations remained unexplained until La Bel and van t' Hoff, In 1S74. simultaneous ly and Independently devised a theory which has up till now stood the test of re search. It 13 briefly this: Imagine two regular tetrahedra, or three-sided pyra mids, standing each on Its triangular base. An idea can best be got by a model, easily made by laying on a table three luclfer matches so as to form an equilateral tri angle, and erecting a tripod with thrca other matches, so that each leg of tha tripod stands on one corner of the tri angle. At the centre of such a tetrahe dron, an atom of carbon Is supposed to be placed. Marsh gas, CIT,, is supposed to have such a structure, each corner, or solid angle oftho structure (of which there are four; being-occupied by an atom of hydrogen. Tills represents the solid or stereochemical formula of methane, or marsh gas. Now suppose one of lhe atoms of hydrogen in each of these structures to be replaced by chlorine, the group (OH), 'J vsl i vl 1 r f. ID